Add configuration for logout handler

Currently, setting `user_context: enabled` will throw an exception if no proxy client is configured because of the logout handler, which requires the client. This PR solves that by adding a `user_context.logout_handler.enabled` config option, which defaults to auto, so won't be available without a proxy client.
But the logout handler really only does something it is added to the Symfony firewall. So does it really make sense to set it to `false`?